The invention relates to hydroxyl-containing graft polymers based on a graft base, which contains a polymer comprising diol components and diisocyanate components which has at least two urethane groups in the molecule, onto which graft base have been grafted polymer chains comprising units of vinyl esters of carboxylic acids, having 3 to 20 carbon atoms, and/or hydrolysis products thereof, and if desired units of other ethylenically unsaturated monomers and/or hydrolysis products thereof.
The compounds of the type described have been disclosed in German Patent Application P 37 32 089.0. The introduction of ester groups into polymers of this type is described, for example, in Houben-Weyl, Methoden der organischen Chemie [Methods of organic Chemistry], volume 14/2, page 723 et seq. (1963).
The modification of hydroxyl groups in polymers likewise belongs to the prior art. In order to carry out this modification, the said polymers are initially reacted with acylating reagents such as for example esters, carboxylic acid chlorides or carboxylic anhydrides. By virtue of this further reaction of the hydroxyl groups of the polymer, it is possible to tailor the solubility of the latter in different solvents.
The reaction as such can be carried out in different ways. According to DE-A 3,144,905 (=U.S. Pat. No. 4,517,277), a polyvinyl alcohol suitable for use as a binder for a light-sensitive recording material is reacted with a large excess of an acylating agent containing polymerizable groups in an aprotic dispersing agent. The disadvantage of this process is that it is necessary to use a large excess of acylating agent, and also that it is not possible to carry out the reaction in solution, but only in suspension.
DE-C 1,522,359 (=U.S. Pat. No. 3,427,161) describes a corresponding reaction of a copolymer of vinyl acetate and vinyl alcohol with methacryloyl chloride in pyridine. In this case, pyridine is both catalyst and reaction medium.
DE-A 3,322,994 (=U.S. Pat. No. 4,554,240) discloses that the excess of acylating agent can be reduced, for example to 1.1 to 1.5 times the molar quantity, by using amines as catalysts, in particular p-aminopyridines. However, the reaction is nevertheless carried out in heterogeneous phase, i.e. in an aprotic dispersing agent.
DE-A 3,541,162 (=AU 86/65535) likewise describes a reaction of polyalkylene oxide-vinyl ester graft copolymers with methacrylic anhydride in the presence of an amine as catalyst, on this occasion: p-dimethylaminopyridine. In this case also, it must be assumed that a heterogeneous reaction is involved - the reaction is carried out in dimethylformamide. Also the starting material is merely swollen in the reaction medium.
Although the excess of carboxylic acid derivatives can be reduced by using the catalysts previously described, there is a great, additional disadvantage with all these catalysts: the products arising from these reactions are colored and therefore have only limited use as binders for light-sensitive mixtures, and even then only with the toleration of great disadvantages.
It is apparent, among other sources, from DE-A 3,534,476 (=U.S. Pat. No. 4,778,847) that the coloration of the products, which is known to those skilled in the art, is due to the catalysts. In this publication, instead of the amino compounds hitherto used as catalyst, a certain imidazole is proposed. Although the coloration of the product is intended to be reduced by the use of this catalyst, a certain coloration is still present according to the reported particulars. Moreover, the practical use of this specific catalyst is less advantageous, since it is relatively expensive. Likewise, the examples given in this publication show that a heterogeneously conducted reaction is involved, since the reactants are merely dispersed in the reaction medium.